Currently, there are three types of gas station fuel vapor recovery technologies which can be categorized as primary recovery, secondary recovery and tertiary recovery systems:                (a) Primary recovery system refers to, as an fuel tank truck is discharging fuel into a fuel depot of a gas station, fuel vapor which is discharged from the fuel depot is recovered;        (b) Secondary recovery system refers to, as fuel gun is used to fill an automobile fuel tank with gasoline, fuel vapor discharged from the fuel tank is recovered; and        (c) Tertiary recovery system generally refers to a recovery device being installed in the fuel depot to carry out fuel vapor separation on extracted fuel vapor, discharge air and liquefied fuel.        
As the gas station-fuel vapor recovery standard of our country is becoming more and more strict, secondary fuel vapor recovery facilities start to become popularized and installed in gas stations. Currently, gas station fuel vapor recovery facilities that have high market shares all belong to foreign brands. According to models for controlling the fuel vapor amount/the fueling amount ratio (fuel vapor recovery ratio, A/L), there can be the following four types:                (1) Model of controlling regulating valve by fuel vapor flow: This model utilizes flow velocity during fueling to generate pressure to control the vapor intake amount of the regulating valve in order to achieve the fuel vapor recovery ratio of about 1:1. This model is adopted by Healy company and Elaflex company.        (2) Pulse sensor model adopting flow metering: This model utilizes a signal of a flow sensor during fueling to control a variable-frequency motor or adjust the opening degree of a proportional valve. When fuel flow is large, the voltage signal frequency of the pulse sensor increases, the rotational speed of the variable-frequency motor increases or the opening degree of the proportional valve increases accordingly. Such a model can achieve the fuel vapor recovery ratio of about 1:1. This model is adopted by fuel vapor recovery equipment manufacturers including companies such as Gilbarco in USA, Tokheim in Europe, Wayne and NP.        (3) Post-processing model: This model utilizes a vacuum pump to pump a great deal of air-containing fuel vapor adopting (1.4:1) to (2.4:1) during fueling back into a fuel tank, and, after the vapor pressure of the fuel tank is increased, the redundant fuel vapor is sent into a combustion tower and burnt, or is recovered by adsorption, or a membrane separation recovery device is installed. Main equipment suppliers include major companies in Europe and USA, such as Hasstech, hirt and OPW.        
The above-mentioned fuel vapor recovery systems have the following defects:                (1) Since the saturated vapor pressure of easy-to-gasify media, such as gasoline, can increase significantly as temperature increases, the easy-to-gasify media, such as gasoline, will become more volatile at higher temperatures. Therefore, it would be unreasonable for the previous two models to maintain the fixed vapor liquid ratio of 1:1. On the contrary, if the vapor liquid ratio is high, using the post-processing mode, as an example, will result in fuel vapor loss in the fuel tank, and moreover, electrical energy will be wasted as well if the recovery ratio is too high.        (2) In the mode of regulating fuel vapor recovery flow by means of the proportional valve, the opening degree of the proportional valve is adjusted by a spool, and the position of the spool of the proportional valve will get loose after the proportional valve is used for a period of time; moreover, an on-site worker may adjust the screw of the spool of the proportional valve without permission, and as a result, the performance of the proportional valve can be changed, causing a nonlinear change in the vapor liquid ratio of the system.        (3) For the mode of regulating fuel vapor flow by means of the proportional valve or the mode of controlling the rotational speed of the motor of the vacuum pump only by the pulse frequency of flow metering, the system defaults the vapor recovery amount of the vacuum pump to be a constant; because the phenomena of vapor recovery amount decrease and insufficient suction exist in the vacuum pump of the on-site fuel vapor recovery system, these will cause great influence on the vapor liquid ratio of the system, and the system cannot be adjusted adaptively to achieve a correct vapor liquid ratio.        
All the above-mentioned fuel vapor recovery systems do not have a real-time vapor liquid ratio display function, so that on-site workers may not be able to visually track of the working conditions of the systems, bringing difficulty to system maintenance.
In the process of transferring and metering fuel, especially gasoline, biofuel ethanol gasoline and other easy-to-gasify media, the fluid medium flows in a pipeline in the form of both vapor and liquid. The change of the physiochemical properties of the transferred medium (for example, the biofuel ethanol gasoline is easier to gasify) will inevitably affect the vapor liquid separation property in the process of pumping, and fuel vapor recovery control parameters will also need to be changed accordingly. The determination of the vapor liquid ratio in fuel vapor recovery is closely related to the process of transfer, and depends on factors such as pipeline conditions, fuel product properties, temperature and pressure condition in the process of transfer. Especially, temperature has significant influence on the volatility of easy-to-gasify media. According to a test, the environmental temperature of gas stations in our country is about 0 to 40° C. in general, and under this temperature condition, the variation range of the vapor liquid ratio of fuel vapor volatilization is about 1-1.4.
Based on the conditions of gas stations in our country, the above-mentioned fuel vapor recovery systems still have room for optimization, and the whole recovery systems can be made simpler and more efficient.